This invention relates to communications cables in general and more particularly to an improved method of twisting such cables.
In the manufacture of balanced communications cables, i.e., of cables whose cores are composed of conductors twisted to form pairs or quads, electrical decoupling of the transmission circuits formed by the conductors is, as a rule, done by the manner of twisting. this is accomplished primarily by means of different twists for the individual twisted units such as pairs or quads as well as for the twisted groups formed by these twisted units, such as bundles or twist layers, and also by changes in the twist during the twisting of the twisted units or twisted groups.
Reduction of the electric coupling is obtained in conventional twisting through the use of rotating feeds or rotating take-up devices as well as through the SZ twisting method which has recently been developed and in which the twisting elements are twisted in successive longitudinal sections alternatingly with a left-hand twist (S-twist) and a right-hand twist (Z-twist. Twisting of this kind has an advantage that the elements to be twisted can run off from stationary frames and that the further processing of the SZ-units formed by the twisted elements can follow in the same serial operation. In general, two twisting operations which heretofore have been performed separately, e.g., the twisting of quads to form a base bundle and of base bundles to form a main bundle, but more significantly the twisting of conductors or elements to form quads and the twisting of the quads to form a base bundle, can be combined into a single operation.
In principle, it is also possible, by means of rotating run-offs and/or rotating take-up devices to combine two twisting operations, which have heretofore been performed separately, in a single operation using the conventional twisting method. However, in such a case it is necessary that the run-offs of the material to be twisted as well as the pull-off and take-up device rotate about the twisting axis (Western Electric, The Engineer, July/October, 1971, pages 50 to 55).
For the SZ twisting, which has been developed in more recent times, rotating twisting devices which contain an intermediate accumulator are typically used. In such devices accumulators with fixed storage content which rotate with a speed or direction of rotation which alternates from section to section, while the running speed of the twisted elements is kept constant, can be used. The accumulators of fixed storage content may also rotate with constant speed and direction of rotation, while the running speed of the twisted elements is changed from section to section. A further possibility is that of increasing or decreasing the storage content of the accumulator alternatingly while leaving the rotary motion of the intermediate accumulator unchanged. In addition SZ twisting devices which operate with stationary intermediate accumulators equipped with flying twisting yokes are known. In addition, SZ twisting methods in which, instead of intermediate accumulators, twisting heads which grip the material to be twisted by friction force from the outside with the material being held in a stretched condition within the twisting apparatus are known. In such apparatus either the rotary motion, the physical position or the force transmission of one or more twisting heads or the running speed of the twisted material or the length of the twisting sections formed between the twisting heads and the corresponding twisting points are periodically changed (see the magazine "Draht", vol. 22, (1971) No. 9, pages 619 to 625). Thus, a common factor in all these known SZ twisting methods is that one or more of the process parameters determining the length of lay of the SZ twist units produced, e.g., the speed or direction of rotation of the twisting device or the pull-off speed of the twisting element or the rate of change of the storage content, are changed or reversed at intervals.
In the SZ twisting of conductors to form twisted units, the electrical coupling is detrimentally affected by the reversal points of the twisting direction, in the vicinity of which the conductors are not twisted or only slightly twisted. In order to counteract this effect, a method of arranging the reversal points of the twist direction so that it is staggered in adjacent twist units has been proposed. Furthermore, variation of the lay of the SZ twist and/or the reversal points of the twisting device continuously by a given amount has been suggested in German Auslegeschrift No. 2,213,693. In this manner systematic, i.e., length-proportional, coupling is converted into primarily stochastically distributed couplings which thus increase with length according to a root law, and detrimental coupling is thereby reduced.
Although these methods work well undesirable coupling is still present in the cables. Thus, it is the object of the present invention not only to reduce the effects of systematic coupling in communications cables but to eliminate these effects as completely as possible.